Somatosensory Science Facts

Charles Pidgeon, PhD, Sehej Bindra

• Publisher: MainspringBooks
• Year: 2021
• Print ISBN: 9781641336406
• eBook ISBN: 9781641336420
• Topic: Education
• Subtopic: Inclusive Education

Tactile sensations are well studied, and much is known about this sensorysystem. The complexity is difficult to learn because of the breadth ofinformation available. The approach of this book was to inform interestedreaders about this system through questions and answers. For instance, understanding the science causing people not to be able to tickle themselvesis insightful. There are approximately 200 questions and answers for thereader to think about on their own pace. Questions include a thoroughdescription of Sensory Systems, Sensory Receptors, Cognitive and Corticalprocessing, Stimuli (pain, heat, pleasure, etc.), Meditation and brain Waves, Social Behavior, and much more.

Part II:

Sensation

1 Touch and Pressure

1. What is the evolutionary importance of touch?
   Touch has numerous primitive functions, indicating that tactile perception is deeply rooted in the evolution of animals. Touch is a form of chemosensing, the general sensory ability to detect external chemicals and respond accordingly. If a stimulus is rewarding (such as food) the organism will move towards the stimulus. If a stimulus is threatening (such as a signal that a predator is nearby) the organism will move away. This response of attraction and evasion of stimuli is regulated by primordial brain regions such as the reticular formation of the brainstem, indicating its evolutionary importance since the existence of primitive animals.
   Additionally, tactile perception influences other important physiological functions such as movement, emotion, and social bonding. In fact, touch is the first sense to develop in fetuses. Overall, touch is deeply-rooted in biological history and is essential for an array of basic animal functions.
   Greger, Rainer, and Uwe Windhorst. Comprehensive Human Physiology: From Cellular Mechanisms to Integration. 1996.
   Ackerman, Diane. A Natural History of the Senses. Vintage Books, 1995.
2. What are the types of tactile receptors and where are they located?
   There are 4 major types of tactile receptors located in the skin: Meissner’s corpuscles, Merkel cells, Ruffini endings, and Pacinian corpuscles. Meissner’s corpuscles and Merkel cells, located in the dermal papillae and stratum basale respectively of the epidermis respectively, are responsible for light touch. Meissner’s corpuscles simply consist of a nerve ending with a thin capsule, while Merkel cells are composed of a two-cell system: a cell that senses the touch and a nerve ending that responds to it. Pacinian corpuscles and Ruffini endings are located within the dermis and sense vibration and pressure. These receptors have a capsule over their nerve ending that makes it more difficult to excite. Pacinian corpuscles are fast at adapting to their stimuli, meaning that they will not sense a prolonged stimulus after some time. On the other hand, Ruffini endings are slow-adapting, meaning the receptor continues to respond to a prolonged stimulus as it is applied.
   Piccinin, Meghan A. “Histology, Meissner Corpuscle.” StatPearls [Internet]., U.S. National Library of Medicine, 15 Jan. 2020, www.ncbi.nlm.nih.gov/books/NBK518980/.
   Quindlen, Julia C, et al. “Multiscale Mechanical Model of the Pacinian Corpuscle Shows Depth and Anisotropy Contribute to the Receptor’s Characteristic Response to Indentation.” PLoS Computational Biology, Public Library of Science, 21 Sept. 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4577116/.
   Abraham, Jacob, and Sherin Mathew. “Merkel Cells: A Collective Review of Current Concepts.” International Journal of Applied & Basic Medical Research, Medknow Publications & Media Pvt Ltd, 2019, www.ncbi.nlm.nih.gov/pmc/articles/PMC6385537/.
   Darian-Smith. “Ruffini Corpuscles.” AU Psychology Web Server, Athabasca University, psych.athabascau.ca/html/Psych402/Biotutorials/27/ruffini.shtml.
   
   Various mechanoreceptors are located in different regions of the skin.
3. What is a Meissner’s corpuscle and how does its structure aid its function?
   Meissner’s corpuscles are a type of mechanoreceptor present throughout the body in dermal papillae—“bumps” that protrude from the dermis into the epidermis of the skin—and form a coiled spring-like pattern when viewed under a microscope. The corpuscles are distributed along the skin with high concentrations in the fingertips, the primary organs used to feel objects.
   Composed of flattened cells that are densely stacked on top of each other, Meissner’s corpuscles are responsible for the perception of fine touch and vibration. Once stimulated, they fire towards the postcentral gyrus (the structural location of the primary somatosensory cortex) in the brain, where these signals are interpreted as fine touch. Meissner’s corpuscles are easily stimulated by any deformation since the nerve ending has minimal covering. This sensitivity allows the corpuscle to function in fine touch.
   Cauna, Nikolajs, and Leonard L. Ross. The Fine Structure of Meissner’s Touch Corpuscles of Human Fingers. Journal of Biophysical and Biochemical Cytology, 1960, www.ncbi.nlm.nih.gov/pmc/articles/PMC2224947/pdf/467.pdf.
   Piccinin, Meghan A. “Histology, Meissner Corpuscle.” StatPearls [Internet]., U.S. National Library of Medicine, 15 Jan. 2020, www.ncbi.nlm.nih.gov/books/NBK518980/.
   Mancini, Flavia, et al. “Fine-Grained Nociceptive Maps in Primary Somatosensory Cortex.” Journal of Neuroscience, Society for Neuroscience, 28 Nov. 2012, www.jneurosci.org/content/32/48/17155.
   Dartmouth, Dartmouth University, www.dartmouth.edu/~anatomy/Histo/lab_3/neuro/DMS033/popup.html.
   
   Meissner’s corpuscles are located in the upper layer of the dermis where they function to sense light touch and vibration. The corpuscle’s nerve fiber is myelinated.
4. What is a Merkel cell and how does its structure aid its function?
   Merkel cells are clear pressure and position sensitive receptors located primarily in the stratum basale (the deepest layer of the epidermis). Merkel cells are composed of large myelinated nerve endings that release neuropeptides in response to strong pressure stimuli, such as the pain felt when pushed against a wall forcefully. The high density of these receptors (mostly in the palm of hands, soles of feet, face, lips, and mouth) is responsible for the high sensitivity of these body regions. Furthermore, the Merkel cell is a fine touch receptor, which means it stimulates the somatosensory cortex, similar to Meissner’s corpuscles.
   Unlike other mechanoreceptors, which have their ending and axon in the same cell, Merkel cells have a separate axon that detects changes to the Merkel cell to detect tactile input. In comparison to Meissner’s corpuscles and Pacinian corpuscles, Merkel cells are not encapsulated and are responsive to low-frequency vibrations (10-15 Hz). On the other hand, these discs are similar to Meissner’s corpuscles in that Merkel cells are located at the junction between the epidermis and dermis.
   Halata, Z, et al. “The Merkel Cell: Morphology, Developmental Origin, Function.” Casopis Lekaru Ceskych, U.S. National Library of Medicine, 20 Jan. 2003, www.ncbi.nlm.nih.gov/pubmed/12693290.
   Chang, Weipang...

Table of contents

• List of Contributors
• Preface
• Introduction
• General Neuroscience
• Sensation
• Cognition and Cortical Circuitry
• Glossary